1. Field of the Invention
The present invention relates to improvement in the treatment of municipal and/or industrial wastewater by an activated biomass process designed for effective removal of phosphates, nitrogen and BOD/COD from a wastewater stream.
2. Prior Art
Several processes were disclosed in the past regarding removal of phosphate and nitrogen from wastewater. One group of such processes is based on a denitrification stage before the main aeration stage. In order to achieve denitrification, anoxic conditions must be maintained in this stage. One form of such a system is that described by Barnard, J. L. "Water and Waste Engineering" (1974) 33, and subject of South African Patent No. 7205371. In this so-called Bardenpho process, four completely mixed activated sludge basins are operated in series followed by a clarifier in which sludge is returned to the first basin. The first and third basins are operated under anoxic conditions in order to achieve denitrification. These basins receive mixed liquor containing nitrates and nitrites from the second and fourth basins in the series which are aerated. The oxygen content of the return sludge needs to be as low as possible in order to facilitate denitrification in the first zone of the process. Thus, the return sludge characteristics prevent any nitrification in the first stage of the treatment process.
A very similar process was presented by Baenens in U.S. Pat. No. 4,183,810. The proposed zone sequence is denitrification, nitrification, denitrification and final sedimentation. Two feedback streams are admitted to the initial denitrification zone. Regarding the characteristics of the feedback streams, the same applies as with the Bardenpho process.
A process that also features denitrification prior to nitrification is described by Klapwijk in U.S. Pat. No. 4,183,809. The application of this process is advantageous to other prior art only if the ratio between the organic loading (measured in mg/l COD according to the dichromate method) and the total amount of nitrogen (mg/l) is approx. between 3 and 7 (See col. 2, line 45). Due to the design of the denitrification zone prior to nitrification, this process is only capable of removing about 80% of the influent nitrogen concentration (See col. 5, line 5-24). Furthermore, this process can not be used for phosphorus removal.
Another type of process is characterized by a anaerobic/aerobic treatment zone sequence as the main biological treatment train. Such a zone sequence, possibly with an interposed anoxic zone, facilitates phosphate removal in the main flow of the process. The purpose of the anaerobic zone is to set the bacteria under stress and cause phosphate release into the wastewater. This treatment commonly results in higher phosphate uptake in the later aerobic stage and therefore brings about phosphate removal from the wastewater treatment system. An example of such a process is the A/O process as described in U.S. Pat. No. 4,056,465 and in U.S. Pat.No. 4,522,722. A similar process, in a two-stage design, is the AB process as presented by Boehnke et al in U.S. Pat. No. 4,487,697 and U.S. Pat. No. 4,568,462. This process utilizes an initial high-loaded treatment stage followed by a second low-loaded treatment stage. The first stage is a high-loaded adsorption stage with an oxygen content close to zero. The high-loaded operation is necessary for buffering of shock loads of pollutants. A second purpose of the first stage is to crack substances in the wastewater which are difficult to decompose. Activated sludge from an intermediate clarifier is returned to the inlet of the first stage. No sludge recycling is allowed from the second stage to the inlet of the first stage. This stage does not operate strictly as a denitrification or nitrification zone. However, the stage operates facultatively anaerobic. The second stage is a low-loaded aeration stage that serves for nitrification. A clarifier after the second stage functions as a denitrification stage.
Both processes are characterized by recycling anaerobic sludge to the anaerobic stage.
The A/O process achieves phosphate release in anaerobic fully mixed basin with detention times in the order of 1.5 hours.
Another similar process is disclosed in Gregor [European Patent Office No. 0,019,203]. This process is designed in particular for high-loaded organic wastewater with high concentrations of organic nitrogen. The success of the invention is primarily achieved by means of a two-staged activated sludge process. The first stage is highly loaded and a portion of the inflowing wastewater is directly admitted to the second stage. The second stage is the last treatment zone. Furthermore, swim sludge from stage 1 is admitted to stage 2. Stage 2 is operated as a denitrification zone. The success of the invention is achieved at relatively high temperatures of operation [20.degree.-30.degree. C.]. At this temperature, the nitrification efficiency of nitrifiers is higher by a factor 4-5 higher compared to typical temperatures in municipal wastewater treatment.
Another category of processes employs a denitrification stage that follows a nitrification stage [cf.ATV p. 311]. The nitrification is achieved in an aeration zone. From this aerated zone, the wastewater is discharged directly into an anoxic denitrification zone. The wastewater that is discharged from the nitrification zone typically contains a significant concentration of dissolved oxygen. This dissolved oxygen hampers the denitrification in the anoxic zone. Consequently the speed of the denitrification reaction is slow and large treatment volumes are required for sufficient total denitrification. A process of this category has been invented by Parker [U.S. Pat. No. 3,953,327]. Parker increases the denitrification volume efficiency by adding methanol or a comparable carbon reagent.
Levin [U.S. Pat. No. 3,654,147] disclosed a similar process with the following zone sequence: Nitrification, denitrification, nitrification, final sedimentation. Recycled sludge is fed into an aerated zone for nitrification. Aeration is necessary for nitrification because the recycled sludge has an insufficient oxygen content. The recycled sludge is admitted after the primary settling tank, and thus it does not use this and other vessels, tanks, etc. prior to the biological treatment stages for nitrification.